In the aforementioned application Ser. No. 798,686, there is disclosed a gate valve structure primarily utilized in mains carrying fluid under pressure, such as mains in a water distribution system, although it may be used in systems carrying other fluids. The valve seat surrounding one of the ports in the valve casing lies generally in a plane converging at an acute angle to a plane normal to the axis of the through-bore, the valve seat having an upper planar portion facing the other of the ports and merging into a lower portion which forms an unobstructed portion of the through-bore. This type of valve seat provides a face-seal with the resilient seal member of the gate member wedging against the upper portion, an edge-seal with the seal member engaging with the portion of the valve seat forming the through-bore and a combination face and edge-seal with the portions of the seal member engaging the portions of the seat merging from the upper planar surface to the lower portion. In machining such a valve seat in the valve casing, the seat lying in a plane at an acute angle to a plane normal to the axis of the through-bore, a maximum radial thickness is obtained in the planar surface in a vertical plane of the axis of the through-bore which bisects the planar portion. The radial thickness of the planar portion progressively decreases on both sides of the maximum radial thickness to a reduced radial thickness where the intermediate portions of the seat begin. A button-shaped tool is utilized to machine the flat face or planar surface of the seat, and it forms an outer arcuate edge in the cast material of the casing where the material falls away from the outer perimeter of the seal. Narrow corners are created at the junction points where the planar seat portion starts to merge with the intermedite portions.
The gate member for the gate valve structure of the aforementioned application includes an annular resilient seal which is generally rectangular in radial section, the annular resilient seal having a bulbous portion extending axially from the gate member for engaging the face-seat or planar portion of the valve seat, the bulbous portion of the seal member having a maximum axial thickness adjacent its upper portion in a plane extending through an axis of movement of the gate member. The bulbous portion merges into the rectangular lower portion of the seal member, the merging portions of the resilient seal member providing the combination face and edge-seal with the valve seat.
While the above-described gate valve structure of the aforementioned application Ser. No. 798,686 has proved extremely satisfactory for gate valves used in small diameter mains, difficulties have been encountered in situations where the mains are 10" in diameter or greater and the pressure of the fluid is higher. In this respect, it has been found that the bulbous portion on the seal which meets the flat machined surface at locations where it has a minimum radial thickness adjacent the narrow corners created at the junction locations, there is insufficient material in the seal member making contact when the valve is closed and leakage occurred when pressures of the fluid in the main were high.